Since the development and fabrication of the first integrated circuit (“IC”), also know as a “microchip,” back in the early 1970, integrated circuits have become essential components in also every device, product and system produced by the human race. As the number of applications for integrated circuits has increased (ranging form computing and control systems, analog and digital signal conditioning and processing, and data storage), so has their complexity. Because of their complexity, modern day integrated circuits, such as the non-volatile memory (“NVM”) integrated circuit shown in FIG. 1, may include tens of millions of transistors organized into tens or hundreds of related and interconnect circuits and/or circuit blocks.
The NVM circuit shown in FIG. 1 includes an array of NVM cells, an analog circuit block, a digital logical circuit block, and a power-up circuit block. The analog circuit block may include charge pumps and sense amplifiers needed to program/erase and read the NVM array. The digital logic circuit block may include a controller adapted, among other things, to: (1) coordinate the flow of data between an external interface and the NVM array, (2) multiplexers for accessing specific rows and columns of the NVM array, and (3) control logic to coordinate the operation and monitor various analog circuits, such as charge pumps and sense amplifiers, in the analog circuit block. Many circuits and/or circuit blocks within an IC, such as exemplified by the NVM circuit shown in FIG. 1, require different supply voltage levels to operate properly. Thus, a power-up circuit segment may monitor the supply voltage being applied to an IC and may provide an enable/reset signal to one or more of the circuits or circuit blocks when the supply voltage reaches a respective circuit's or circuit block's required voltage level. The power-up circuit may also provide an accurate reference voltage to be used by enabling circuitry associated with each of the circuits or circuit blocks.
Turning now to FIG. 2, there is shown a power-up circuit segment 200 according to the prior art. According to the exemplary prior art circuit of FIG. 2, during power-up, while VDD beings ramping upward, a power reset circuit block 210 provides an enable signal to a comparator 220 once the power reset circuit block 210 determines that VDD has reached a sufficiently high voltage level for the comparator 220 to be reliably operative. The comparator 220 may receive as an input on a first terminal some fraction of VDD, where the fraction is set by a voltage divider 240. On a second terminal, the comparator 220 may receive a reference voltage, where the reference voltage may be set according to the threshold voltage (e.g. 0.4V) of a transistor 230. According to the power-up circuit of FIG. 2, once VDD reaches some multiple (defined by the voltage divider) of the threshold voltage of transistor 230, the comparator may output a bandgap enable signal, which signal is intended to activate a bandgap reference circuit 250. The output of the bandgap reference may be used as an accurate reference voltage for determining when other circuits or circuit blocks may be enabled.
Because a bandgap circuit 250, such as the one shown in FIG. 2, requires a certain supply voltage level (e.g. 1.4 volts) to operate properly, the voltage divider 240 and the transistor 230 threshold voltage may be selected such that the comparator may enable the bandgap circuit 250 once VDD reaches that certain supply voltage level (e.g. 1.4 volts). However, due to the fact that a transistor's 230 threshold voltage may fluctuate up or down based on a number of parameters, including fabrication process deviations and operating temperature, the voltage level at which the comparator 220 may enable a bandgap reference circuit 250 may deviate by several hundred millivolt, up or down. This deviation may cause the bandgap reference operate improperly and may cause other circuits or circuit blocks to be enabled when VDD is below their respective nominal operating voltages.
There is a need in the field of IC design for a power-up circuit and method to provide a relatively accurate reference voltage and/or to facilitate circuit/circuit-block enabling signals.